For the purpose of imparting additional functions to a polymer material while maintaining its performance, introduction of a small amount of reactive functional groups to the polymer compounds has conventionally been made. For styrene-hydrogenated diene block copolymers having high performance as thermoplastic elastomer, attempts to introduce reactive functional groups thereto have been made and there has been known a technology to introduce reactive functional groups such as hydroxyl groups, acid anhydride groups and boronic acid groups in order to improve, for example, the compatibility and adhesiveness to other resins. Of the reactive functional groups to be employed for this purpose, a boronic acid group and boron-containing groups similar thereto are superior to other functional groups because of their high reactivities.
Japanese Unexamined Patent publication No. 7-25198 (U.S. Pat. No. 5,571,871) discloses a styrene-hydrogenated diene block copolymer having a boron-containing group, such as a boronic acid group, at its terminal. Japanese Unexamined Patent Publication No. 6-340783 (U.S. Pat. No. 5,466,748) discloses a resin composition comprising the above described styrene-hydrogenated diene block copolymer and an ethylene-vinyl alcohol copolymer (hereinafter, abbreviated as EVOH). In these publications, what is disclosed as a method for introducing a boron-containing group into a styrene-hydrogenated diene block copolymer is a method of adding that group to an anion polymerization terminal.
Further, Japanese Unexamined Patent Publication No. 9-263608 discloses a method for producing a boron-containing thermoplastic resin, the method being characterized by allowing a boron compound having a boron-hydrogen bond to react with a thermoplastic resin having double bonds in an amount of not less than 0.0001 meq/g in an extruder and subsequently adding a compound having one or two hydroxyl groups in the molecule and having a boiling point of not higher than 300° C. at 760 mmHg. Resins that can be employed as the above described thermoplastic resin having double bonds are exemplified by butadiene-styrene block copolymers, isoprene-styrene block copolymers, and their hydrogenated products.
EVOH is a material excellent in gas barrier properties, oil resistance and flavor retaining properties. However, it is difficult to use EVOH singly due to its drawbacks such as a great moisture permeability and high expense and EVOH is usually used in the form of a laminate structure with a thermoplastic resin such as polyolefin, polystyrene, polyester and polyamide. However, the adhesiveness between these thermoplastic resins and EVOH is poor. Even if EVOH is formed into a laminate structure using a conventional adhesive such as a polyolefin (e.g., polyethylene, polypropylene, and an ethylene-vinyl acetate copolymer) modified with maleic anhydride, ethylene-ethyl acrylate-maleic anhydride copolymer, delamination between layers may occur.
For solving this problem, Japanese Unexamined Patent Publication No. 7-329252 discloses a laminate structure having an EVOH layer and a thermoplastic resin layer laminated through a layer made of a thermoplastic resin containing a boron-containing group such as a boronic acid group. Examples of the thermoplastic resin having a boron-containing group to be used here include hydrogenated products of styrene-isoprene-styrene block copolymers. Further, Japanese Unexamined Patent Publication No.2001-164059 (European Patent No. 1090953) discloses, as an EVOH resin composition with excellent interlayer adhesion, an EVOH resin composition containing an alkali metal salt in an amount of from 50 to 500 ppm in terms of metal element.
However, in the method of addition to a terminal of a polymer, the amount of boronic acid groups to be introduced is limited. Therefore, an effect of introducing the functional groups may be exhibited insufficiently when this method is applied to polymers of high molecular weight. Further, also in the method of addition to an olefinic double bond, application of this method to ordinary styrene-hydrogenated diene block copolymers results in the same problem due to a very small amount of the remaining olefinic double bonds. There is an idea of employing a copolymer containing unhydrogenated diene blocks for avoidance of this problem. In such a case, however, a great amount of boronic acid groups are introduced to allow polymers to gelate and, as a result, the polymers will become unsuitable for use.
Use of a thermoplastic resin having a boronic acid group or the like as an adhesive requires somewhat high cost. An attempt, for cost reduction, to substitute a composition diluted with a thermoplastic resin having no such functional groups will result in insufficient interlayer adhesion between an EVOH layer and a thermoplastic resin layer. Further, in co-extrusion using a styrene-hydrogenated diene block copolymer containing a boronic acid group or the like as an adhesive layer together with other resins, it is difficult to extrude them with stability since the block copolymer is an elastic material. On the other hand, when EVOH contains an alkali metal salt, the thermal stability at high temperature of the EVOH is deteriorated and, therefore, problems such as defective appearance, e.g., coloring, fish eyes and streaks and odor caused by decomposed gas become prone to arise, for example, when such EVOH is formed into a film.
Thus, an object of the present invention is to obtain a highly reactive styrene-hydrogenated diene block copolymer with a sufficient amount of boron-containing groups introduced. Further, another object of the present invention is to provide a laminate that is excellent in gas barrier property and in thermal stability at high temperature and that causes no delamination between layers.